Disable Travis-CI notifications on Slack

We see the Travis-CI status in GitHub pull requests anyway.

.travis.yml

@@ -16,6 +16,3 @@ script:
   - py.test
   - MUTALYZER_TEST_DATABASE_URI=postgres://postgres@127.0.0.1/mutalyzer_test py.test
   - MUTALYZER_TEST_DATABASE_URI=mysql://travis@127.0.0.1/mutalyzer_test?charset=utf8 py.test
-notifications:
-  slack:
-    secure: "VsB4CPZCfUPpnt7sL94mYrws2B9VthQhY1+u7NBm6QkAopel0m1rx1vWx3gQCTR0t9vWiP+pSybtQ81PQFNrOSWDsb1QlyEK1JF1QUw5Zui+5wS8fwFM/sgwGIECuo6Oj0zCwC9KMM4olYEMC2c7TNHv4HgwWiMqEK9ItEYSLQY="